tmac: introduce real manual reference macro instead of overloading IR

The overloading of IR emits magic \X'...' sequences that turn into HTML manual links.
But not all such IR invocations should be manual links;
those had to be written to avoid the IR macro before.
Worse, the \X'...' ending the IR causes troff to emit only a single space after a period.

Defining a new IM macro for manual references fixes both problems.

Fixes #441.

man/man1/git.1

@@ -1,5 +1,5 @@
 .TH GIT 1
-.SH NAME 
+.SH NAME
 git, hg, cvs, codereview \- introduction to using plan9port Git repository
 .SH SYNOPSIS
 .B git
@@ -17,7 +17,7 @@ git, hg, cvs, codereview \- introduction to using plan9port Git repository
 .I path ...
 ]
 .PP
-.B gitk 
+.B gitk
 .PP
 .B web
 .B https://9fans.github.io/plan9port
@@ -37,13 +37,13 @@ which it will create.
 After
 .B git
 .BR clone ,
-the other commands listed 
+the other commands listed
 should be run within the
 .B plan9
 directory tree.
 .PP
 Git downloads the entire revision history
-of Plan 9 from User Space 
+of Plan 9 from User Space
 in addition to the current tree.
 .PP
 .I Git
@@ -55,7 +55,7 @@ current file tree.
 .I Git
 .I diff
 runs Unix's
-.IR diff (1)
+.IM diff (1)
 to compare the files in the local tree with the corresponding
 files in the revision history.
 The special revision
@@ -76,7 +76,7 @@ directory containing Git local repository
 list of files and wildcards to exclude from Git operations
 .SH SEE ALSO
 Unix's
-\fIgit\fR(1),
+.IR git (1),
 .HR http://git-scm.com/doc
 .PP
 .HR https://9fans.github.io/plan9port/